1. Field of the Invention
The present invention relates to a surface texture measuring device such as a coordinate measuring machine which measures displacement using a contact piece, a surface texture measuring method, and a program.
2. Description of the Related Art
In a contact measurement system using a contact probe (a contact piece), it is necessary to inspect any deformation (wear, damage, etc.) of the shape of the tip of the contact piece in use. In inspecting deformation of the shape of the tip of the contact piece, there are the following two methods. In the first method, the tip of the contact piece is inspected by microscopic observation. In the second method, a certain reference workpiece is measured and the result of the measurement is used for inspection. At present, however, measurement is often carried out in an environment in which a microscope cannot be used, and the second method is used instead for inspection.
A detailed explanation of the second method will now be given. First, the measuring device brings the contact piece into contact with a reference workpiece, moves the contact piece along the reference workpiece, and thereby acquires a pseudo measurement point sequence. A pseudo measurement point sequence is a sequence of discrete points sampled from a locus of the center point of the contact piece obtained during a measurement. Pseudo measurement point sequence is described in, e.g., JP2002-357415A.
Next, the measuring device calculates a certain roughness parameter (evaluation value) from the pseudo measurement point sequence and determines whether or not to replace the contact piece based on the roughness parameter (replacement determination). Specifically, replacement determination is executed based on comparison between the certain roughness parameter at present and a nominal value. The nominal value represents the certain roughness parameter obtained when measurement is carried out using an un-deformed contact piece. When there is a great difference between the roughness parameter obtained from the measurement of the reference workpiece and the nominal value, it is determined that the contact piece should be replaced because it is deformed. In some cases, replacement determination is executed based on comparison between the certain roughness parameter at present and the certain roughness parameter in the past. Furthermore, there is a proposal for improvement of inspection accuracy based on measuring a reference workpiece sensitive to change of the shape of the contact piece and a reference workpiece insensitive to such change, and using the ratio between their roughness parameters as an evaluation value. In this way, conventional replacement determination is made based on a certain roughness parameter obtained by measuring a reference workpiece by a contact piece.
However, the amount by which the roughness parameter changes along with deformation of the contact piece fluctuates depending on the surface texture of the workpiece to be measured for obtaining the roughness parameter. Therefore, in the conventional replacement determination, even when the amount of change of the certain roughness parameter obtained by measuring a reference workpiece is equal to or smaller than a specified value (i.e., even when determination is made against necessity of replacement), an evaluation value based on the roughness parameter obtained by measuring an actual measurement target workpiece might not satisfy a criterial value. That is, conventional replacement determination cannot accurately specify such deformation of the contact piece that would affect measurement of an actual measurement target workpiece, and hence cannot correctly determine whether or not to replace the contact piece.